The invention relates to a method for generating the working clock in a module of a data transmission system, in particular in a vehicle occupant protection system of a motor vehicle, and to a corresponding data transmission system with a control unit and one or more modules connected to the control unit via a data transmission line.
Customarily, in methods of the generic type the working clock signal is generated in the module by means of a separate resonator, for example a ceramic resonator in the module. Vehicle occupant protection systems, such as airbag control units often have, in this context, a central control unit (ECU) and externally located modules (satellites) containing acceleration sensors. The externally located modules signal, for example, the accelerations in the case of a side impact or they generate accident early-detection information.
The central control unit is also equipped with a separate time base in the form of a resonator. The externally located modules frequently transmit detection signals to the central control unit in a defined pattern in order to signal their correct active operating state to the central control unit. However, the transmission pattern is dependent on the separate time base of the satellite which is generated by the resonator, with the result that the communication runs asynchronously and is therefore susceptible to faults. Also, the time base used in the satellite must have a high degree of precision in order to ensure the correct digital signal processing in the module. Accordingly, the satellite time base thus provided is technically complex.
In addition, it is important that the working clock of the module or modules and the working clock of the central control unit correspond. If the clocks are not exactly synchronized, errors, in particular aliasing errors, may occur in the evaluation of the acceleration signals measured by the module and transmitted to the central control unit. The result is that the frequency of the actually measured acceleration signals may differ from the frequency, determined in the control unit, of the control signals after their evaluation. These incorrect evaluations can, for example, lead to the undesired, unnecessary initiation of protective measures, and are therefore critical.
It is accordingly an object of the invention to provide a method of producing a working clock in the module of a data transmission system and a correspondingly equipped data transmission system, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows generating the working clock in the module with little outlay.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of generating a working clock in a module of a data transmission system, particularly for a vehicle occupant protection system in a motor vehicle, which comprises:
transmitting, with a control unit of a data transmission system, a synchronization clock signal to a module of the data transmission system;
receiving the synchronization clock signal with a frequency-locked and/or phase-locked loop in the module;
generating, with the frequency-locked and/or phase-locked loop a working clock on the basis of the synchronization clock signal; and
controlling an operation of the module with the working clock, including transmission and reception processes.
In other words, the control unit generates a synchronization clock signal which is fed to the module and on the basis of which the module generates it own working clock with the aid of a frequency-locked and/or phase-locked loop. Since the use of a frequency-locked and/or phase-locked loop requires in fact very much less outlay than the use of a separate resonator, the generation of the working clock in the module is possible with little technical outlay.
In addition, there is the advantage that the working clock formed in the module is continuously correlated with the synchronization clock signal supplied by the control unit so that synchronous operation of the control unit and the module can be achieved. Thus, a synchronous connection is made possible between the control unit and the module, which connection has a significantly higher transmission speed and transmission reliability in comparison with asynchronous communication.
The frequency-locked and/or phase-locked loop of the module can be formed by a cost-effective circuit which can be completely integrated.
In accordance with an additional feature of the invention, the synchronization clock signal is generated in the control unit in the form of voltage pulses, and the data are transmitted from the module to the control unit in the form of current pulses. The interface between the control unit and the module is thus configured as a bidirectional interface which acts as a voltage interface in the direction from the control unit to the module and operates as a current interface in the direction from the module to the control unit. As a result, it is possible to ensure that the synchronization clock signal which is transmitted from the control unit to the module can be transmitted completely independently of a data transmission which may be made simultaneously in the opposite direction. Between the control unit and the module there is thus a synchronous and bidirectional transmission link which is capable of full duplex operation. The control unit can in this case, of course, not only transmit the synchronization of clock signal to the module but also transmit additional data, for example control data for the purpose of programming the module.
In accordance with another feature of the invention, the synchronization clock signal is a low-frequency signal, and wherein the generating step comprises subjecting the clock signal to a frequency multiplication in the frequency-locked and/or phase-locked loop and forming the working clock. The synchronization clock signal which is transmitted by the control unit preferably has a frequency below 10 kilohertz, for example 1 to 2 kilohertz. On the basis of this synchronization clock signal, the frequency-locked and/or phase-locked loop generates in the module an internal signal which is fixed in terms of frequency and/or phase and which is then multiplied by frequency multiplication to the working clock of the module, for example 10 to 16 megahertz. The use of a low-frequency synchronization clock signal provides the advantage that the loading of the central control unit by the generation of this signal is correspondingly reduced and also a possibly simultaneously occurring, very much higher-frequency data transmission can be distinguished clearly from the synchronization clock signal, i.e. does not cause any disruption to the signal and thus to the frequency stability of the working clock in the module.
It is a further refinement of the invention and the data transmission system according to the invention can be simplified still further by using the data transmission line not only for transmitting data and synchronization clock signals but also at the same time as a voltage supply line for the module. For this reason, the module does not require a separate voltage terminal but rather can be fed with the supply voltage from the control unit without additional outlay on interconnections. In this case, the module is preferably provided with a voltage regulator for generating a constant module supply voltage.
With the above and other objects in view there is also provided, in accordance with the invention, a data transmission system, particularly for a vehicle occupant protection system in a motor vehicle, comprising:
a control unit with a synchronization clock signal generator generating a synchronization clock signal;
a module with a frequency-locked and/or phase-locked loop;
a data transmission line connecting the module to the control unit;
the frequency-locked and/or phase-locked loop of the module receiving the synchronization clock signal generated by the synchronization clock generator of the control unit, and, based on the synchronization clock signal, generates a working clock controlling an operation of the module, including transmission and reception processes of the module.
In accordance with again an added feature of the invention, the frequency-locked and/or phase-locked loop includes a frequency multiplier for frequency-multiplying a signal formed internally from the synchronization clock signal.
In accordance with again an additional feature of the invention, the control unit. is operated at a given working clock, and the synchronization clock generator is a voltage source actuated in dependence on the working clock of the control unit.
In accordance with again another feature of the invention, the module transmits data to the control unit in the form of current pulses, and the control unit includes a current measuring device connected to the data transmission line for detecting and demodulating the data signals supplied by the module.
In accordance with again a further feature of the invention, the module contains a controlled current source for generating the data signals.
In accordance with again another feature of the invention, the frequency-locked and/or phase-locked loop has an input, and the module includes a comparator having an input connected to the data transmission line and an output connected to the input of the frequency-locked and/or phase-locked loop.
In accordance with a concomitant feature of the invention, the control unit impresses a d.c. voltage on the data transmission line, and the module includes a voltage regulator having an input connected to the data transmission line and an output outputting a supply voltage for the module.
The invention can be used not only in vehicle occupant protection systems such as airbag systems or belt-pretensioning systems but also for transmitting data between. control units provided for other purposes and associated externally located circuits, in particular in motor vehicles. In addition, it is also possible to connect to the control unit a plurality of modules whose internal working clock is generated synchronized in each case on the basis of the synchronization clock signal supplied by the central control unit.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of generating the working clock in a module of a data transmission system, and a correspondingly equipped data transmission system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.